The invention relates to a traveling water screen basket. More particularly, the invention relates to a pretensioned mesh insert for a traveling water screen basket and a method for producing the pretensioned mesh insert.
Power plants, foundries, and other types of factories are often located near a body of water in order to satisfy their critical need for water, which is typically used as a coolant. However, water taken from a natural source must be filtered to prevent naturally occurring debris and aquatic life from being taken in with the water. The filtering process is designed both to preserve the existing aquatic life and to remove from the water anything that might damage the equipment in the plant or factory.
Typically, traveling water screen through-flow filtration units are used to accomplish the desired filtering. The units are placed in the water reservoir in front of the plant intake ports and include a series of "baskets" mounted on an endless carrier chain assembly. The baskets each include an assembly of screens through which water from the reservoir passes on its way to the intake parts. The screens operate to filter the water and are attached to a frame called a basket. The basket includes a lip used to collect debris from the water. The carrier chain is driven by an electric motor causing the baskets on the upstream or ascending side of the unit to travel upward. As the baskets exit the water, they pass through a splash housing where they are cleaned. The baskets then follow the carrier chain to the downstream or descending side of the unit to repeat the cycle in endless loop fashion.
As already mentioned, each basket includes a screen assembly mounted thereon to accomplish the filtering task. The screens known in the prior art are simply bolted to the baskets themselves or to some type of supporting frame which is then attached to the basket. When a selected screen is on the upstream side of the unit, water flowing through the upstream or filter side of the screen creates a first force on the screen from a first direction. This force causes the screen to deflect a given distance in the first direction from its resting non-tensioned position placing the screen in tension and placing stress on the screen at its point of contact with the frame or basket. The carrier chain assembly then moves the water screen baskets out of the water through the splash housing to the descending side of the unit. When this occurs, the pressure is relieved allowing the mesh to flex back to a resting state. When the screen is again submerged on the descending side, water flows through the screen in a relative direction opposite to the flow through the screen when the screen is on the ascending side. The reversed orientation of the screen causes the flowing water to place a force on the screen in a direction opposite to that existing on the ascending side of the unit. This reversal of the force vector causes the screen to move from the point of deflection experienced on the ascending side of the unit through the resting non-tensioned position to the point of deflection in the opposite direction where the screen is again forced into tension and again placing stress on the screen at the point of contact with the frame. The constant series of transitions from deflection in one direction to deflection in the opposite direction and vice versa causes the material comprising the screens to fatigue quickly at the point of contact between the frame and the screen. This causes the screen to fracture and bend away from the frame resulting in the introduction of debris into the cooling water used in the plant or factory. The degree of fatiguing is at least in part related to the degree of travel from deflection in one direction to deflection in the other direction.
A need exists for a screen for a traveling water basket that substantially eliminates the degree of travel during deflection resulting from the force of water in either direction as the screen passes through a cycle of the carrier chain assembly to prevent premature fracture.